1. Field of the Invention
The present invention relates to an acoustic tube that can be used in, for example, a microphone, and a directional microphone using the same, and more particularly to an acoustic tube and a directional microphone, the manufacturing process of which can be simplified and the size of which can be reduced.
2. Related Background of the Invention
For example, a microphone with narrow directivity is configured so that sound waves at angles other than that of the main sound collection axis enter into an acoustic tube through a front surface of the acoustic tube and an acoustic resistance part on the side thereof by arranging the acoustic tube with leakage at a front acoustic terminal of a microphone unit. The sound waves that have entered through the front surface of the acoustic tube and the sound waves that have entered through the acoustic resistance part attenuate due to a difference between both phases and the sound waves along the main sound collection axis are emphasized, and thereby a narrow directivity is obtained.
As an example of a specific configuration of the acoustic tube, there is one having a plurality of through holes for acoustic resistance on a circumferential surface and provided with an acoustic resistance material composed of felt, resin mesh, nonwoven fabric, etc. on the outer circumferential surface of the acoustic tube. The acoustic tube is known which further has an outer case engaged with the outer circumferential surface thereof, the inner diameter of which is greater than the outer diameter of the acoustic resistance and in the circumferential surface of which a plurality of through holes are formed, and has a microphone unit disposed in the acoustic tube (for example, refer to the patent document 1).
However, for the configuration of the acoustic tube described in the document patent 1, it is necessary to further engage an outer case with the outer circumferential surface side because the acoustic resistance material composed of unwoven fabric etc. is provided on the outer circumferential surface of the acoustic tube, thus the structure becomes complex and the manufacturing process becomes intricate. Therefore, there is known a narrow directional microphone that has obviated the need for the outer case by providing the acoustic resistance material on the inner circumferential surface side of the acoustic tube (for example, refer to the patent document 2).
FIG. 2 shows a conventional example of an acoustic tube based on the same idea as that of the acoustic tube described in the patent document 2. In FIG. 2, on the circumferential wall of an elongated, cylindrical acoustic tube 1 made of metal, a plurality of slits 2 constituting an acoustic resistance part in the direction parallel to the center axis line of the acoustic tube are formed in such a manner as to be arranged aligned in the direction of the center axis line, penetrating through the circumferential wall of the acoustic tube 1. There is a case where only one row of the slits 2 is formed or a case where two or more rows, for example, two rows in total are formed, each row being formed on the respective positions of the circumferential wall in opposition to each other. An acoustic resistance material 3 is bonded to the inner circumferential surface of the acoustic tube 1. The acoustic resistance material 3 is made of, for example, resin mesh and by bonding, for example, by using a double-sided adhesive tape, is bonded to the inner circumferential surface of the acoustic tube 1. The right-hand side of the acoustic tube 1 in FIG. 2 is the proximal end and the left-hand side is the front end and to the proximal end portion, a microphone unit, not shown, is attached. The front end side is fitted with a front damper 4 and its outside is fitted with a grill mesh 5 and its further outside is fitted with a grill 6.
[Patent document 1] Japanese Utility Model Application Laid-Open No. Hei 6-48294
[Patent document 2] Japanese Patent Application Laid-Open No. 2000-83292